The rail of a linear-motor electromagnetic train has a massive reinforced-concrete rail beam that is cast normally in situ. On it are a pair of horizontally spaced, parallel, and horizontally oriented support rails and a pair of horizontally spaced, parallel, and vertically oriented guide rails. In operation the train floats, that is suspends itself magnetically, with respect to these rails, and the linear motor of the train pulls the thus suspended train along.
Even with the most careful construction techniques, it is normally impossible to cast the concrete rail beam, which is a massive T-section construction, within tolerances of more than +/-20 mm. For high-speed travel the electromagnetic train needs to have guide and support rails positioned within +/-3 mm of an ideal orientation. This tight tolerance is essential for high-speed travel of a train supported on a magnetic field.
Thus it is standard practice to painstakingly mount the guide and support rails on the reinforced-concrete rail beam by appropriate fasteners and spacers, normally providing a layer of special-mix concrete underneath the rails. Various gauges and alignment devices, basically of the type used on standard wheel-type tracks, are employed for accurately positioning these rails. The job is complicated further because the guide and support rails are very close to each other. Obviously such mounting of the rails is an extremely onerous procedure that adds greatly to the first costs of such a transit system.